In recent years, as concern about environmental pollution has increased, a secondary battery, which can be charged and discharged, has attracted considerable attention as a power source for vehicles in order to solve problems caused by existing gasoline and diesel vehicles using fossil fuel. As a result, electric vehicles (EV), which are operated only using a battery, and hybrid electric vehicles (HEV), which use jointly a battery and a conventional engine, have been developed. Some of the electric vehicles and the hybrid electric vehicles have been commercially used. A nickel-metal hydride (Ni-MH) secondary battery has been mainly used as the power source for the electric vehicles and the hybrid electric vehicles. In recent years, however, the use of a lithium secondary battery, which has high energy density and high discharge voltage, as the power source for the electric vehicles and the hybrid electric vehicles has been attempted. High output and large capacity are needed for such a secondary battery to be used as the power source for the electric vehicles and the hybrid electric vehicles. For this reason, a plurality of small-sized secondary batteries (unit cells) are connected in series with each other so as to construct a battery pack.
A large amount of heat is generated from the secondary batteries, i.e., the unit cells, during the charge and the discharge of the unit cells. When the heat generated from the unit cells during the charge and the discharge of the unit cells is not effectively removed, heat is accumulated in the unit cells with the result that the unit cells are degraded. Especially, the lithium secondary battery has a high possibility of danger in that the lithium secondary battery may catch fire or explode due to high temperature or overcharge of the lithium secondary battery. Consequently, it is very important to secure the safety of a medium- or large-sized battery pack including lithium secondary batteries as the unit cells.
However, currently developed safety systems have problems in that, when abrupt temperature increase, abrupt overcharge, or abrupt overcurrent occurs in a medium- or large-sized battery pack, the safety systems do not effectively control such abnormality. Also, this abrupt circumstantial change may put the safety system out of order. For example, a cooling system serves to circulate a coolant (for example, air) between the unit cells so as to decrease the temperature of the unit cells to a predetermined temperature level. When the temperature is abruptly increased entirely or partially in the battery pack, however, the cooling system does not provide a cooling efficiency sufficient to retrain the abrupt temperature increase. In addition, an operation control system serves to detect the temperature and the voltage of the unit cells and, when the detected value exceeds a predetermined level, interrupt electric current so as to suspend the operation of the battery pack. When the cause is not eliminated only by the suspension of the operation of the battery pack due to the abrupt circumstantial change, however, it is not possible to obtain the desired effect.
Consequently, there is high necessity of a technology for fundamentally solving problems caused when all or some of the unit cells are unstable due to various causes. Furthermore, since the medium- or large-sized battery pack is a battery pack including a plurality of secondary batteries, the fire or explosion of some of the secondary batteries results in the consecutive fire or explosion of the remaining secondary batteries, whereby a big accident may be caused.
In this connection, there has been proposed a safety device constructed in a structure in which a fire extinguishing agent or a coolant is disposed in a battery pack system while the fire extinguishing agent or the coolant is stored in a predetermined container, and, when a detected value exceeds a predetermined temperature level as a result of the detection of the temperature of the battery pack, the fire extinguishing agent or a coolant is injected using an injection unit. However, this safety device needs several components, such as the container for storing the fire extinguishing agent or the coolant, the injection unit, the temperature detection unit, etc. As a result, the manufacturing costs of the safety device are high, and the safety device needs a large installation space in the battery pack system. Consequently, the use of the safety device is not suitable for the manufacture of a more effective battery pack.